In recent years, there is an increasing demand for laser beam printers in accordance with diversification and development of office automating machines because such printers have become available with smaller sizes at lower costs and have the advantage of high speed printing. In the laser beam printer including a deflection device such as a polygonal mirror, there occur tilt errors with respect to a direction perpendicular to a scanning plane (an auxiliary scanning direction). The term "scanning plane" means a plane including a main scanning line on the scanned objective surface and the optical axis of an image projection optical device provided between the deflection device and the objective surface. The light beam deflected by the deflection device having such errors results in an image forming position on the objective surface slipped or deviated in the auxiliary scanning direction and an irregularity in the scanning line pitch.
For eliminating the irregularity mentioned above, U.S. Pat. No. 3,750,189 proposes an optical system for laser printers and the like which includes a beam shaping optical element such as a cylindrical lens for causing a light beam emitted from a light source to form a linear image on the surface of a deflection device, a beam configuring optical element such as a cylindrical lens for restoring the deflected beam into a circular form and a beam converging optical element having f.theta. characteristics for converging the restored beam to form an image on an objective surface at a constant scanning speed. The above-mentioned conventional system has the drawback of reduced freedom for improving distortion characteristics provided for the beam converging optical element to realize the constant scanning speed of the beam and image forming characteristics for forming images on the objective surface. In order to realize excellent distortion and image forming characteristics, it is necessary for the beam converging optical element to increase the number of lenses thereof.
U.S. Pat. No. 3,946,150 discloses a proposal for improving the foregoing conventional construction. In this construction, a beam configuring optical element such as a cylindrical lens is interposed between the converging optical element and the objective surface to be scanned. The beam configuring optical element mentioned above must be located close to the objective surface in order to produce high quality images. It is therefore necessary for the beam configuring optical element to be long in the main scanning direction, although a compact construction is desirable.
Another known construction is disclosed in U.S. Pat. No. 4,379,612 wherein an image projection optical device disposed between the deflection device and the objective surface to be scanned by the beam comprises a single spherical lens and a single lens having a toric surface. This image projection optical device has the distortion characteristics for realizing the constant speed scanning of the beam and further the function to correct the tilt errors of the deflection device in cooperation with a linear image forming optical device disposed in front of the deflection device.
Although this construction realizes a compact optical system, only a limited design freedom is provided thereby. For example, it is difficult to enlarge the field angle for increasing the scanning range of the beam while maintaining excellent distortion characteristics and an outstanding function to correct the tilt errors of the deflection device.